1. Field of the Invention
The present invention relates generally to a hydraulic-pressure control system for two brake lines for use within motor vehicles, and more particularly to a pressure control valve for supplying a limited fluid pressure to one of the brake lines.
2. Description of the Prior Art
In order to cope with the fact that either of the two brake hydraulic circuits, but normally the front-wheel-brake hydraulic circuit, has failed or ruptured, with the result of failure to generate a braking hydraulic pressure, many attempts have been hitherto proposed for a control device which interrupts the operation of another hydraulic pressure control device within the rear wheel brake hydraulic circuit, thereby directly supplying an uncontrolled master-cylinder-hydraulic-pressure to the rear wheel brake circuit.
However, the prior art device dictates the use of a complex bypass circuit in addition to the rear wheel brake hydraulic circuit, whereby in the event of the failure of the front wheel hydraulic circuit, an uncontrolled hydraulic pressure is supplied by means of the aforenoted bypass circuit to the rear wheels.
Within a braking system utilizing disc brakes for the front wheels and drum brakes for the rear wheels of the motor vehicle, it is imperative to prevent the premature actuation of the front-wheel disc brakes prior to the actuation of the rear-wheel drum brakes so as to thereby achieve the safe running of the motor vehicle during the braking period.
Metering valves which have been hitherto used for achieving the aforenoted object, suppress the build-up in the actuating pressure within the front brakes for a predetermined period of time, immediately after the commencement of the braking operation, so as to permit the shoes of the drum brakes within the rear wheels to contact the drums during the aforenoted period of time, thereby bringing the shoes into an operable condition, in an attempt to cope with play within the rear-wheel brakes.
However, in the event of a failure of the rear-wheel braking system, it is mandatory that the actuation of the metering valve normally adapted to temporarily restrain the pressure build-up of the braking-hydraulic pressure within the front wheel, be interrupted, while causing the simultaneous and immediate commencement of the actuation of the front-wheel brakes for ensuring the safety of the motor vehicle.